1. Technical Field
The present disclosure relates to a wiring substrate.
2. Description of Related Art
There have been known wiring substrates on which an electronic component such as a semiconductor chip is mounted. When solder bumps are provided on connection pads on such a wiring substrate, surface treatment layers such as a nickel layer/gold layer are formed on upper surfaces of the connection pads (see e.g., JP-A-2001-60760).
As described in an introduction section for explaining the present invention (see later), when producing the wiring substrate, a solder bump is formed on a connection pad exposed from an opening portion of a solder resist via a nickel layer. In this case, tin contained in the solder and copper contained in the connection pad are diffused through a gap between the nickel layer and an inner wall of the opening portion of the solder resist, and an intermetallic compound including tin, copper and the like is formed on a peripheral area of the nickel layer.
Such an intermetallic compound is mechanically brittle and thus easily damaged by external stress, which may lead to a connection failure of the solder bump.